1. Field of the Invention
The present invention relates to an image forming apparatus, such as an electrostatic copying machine, a laser beam printer, etc., which forms an image according to an electrophotographic process.
2. Description of the Related Art
In an electrostatic copying machine, a copy image is formed as in the following manner. First, a surface of an original is illuminated and scanned, and a photoconductor drum is rotated synchronous with the illumination and scanning. At this point, the photoconductor drum is exposed to reflected light from the original. A surface of the photoconductor drum before exposure is electrostatically charged uniformly, and it is selectively discharged by exposure to form an electrostatic latent image corresponding to an original image.
The electrostatic latent image thus formed is developed into a toner image by a developing device, and the toner image is transferred to a copy sheet through corona discharge caused by a corona discharger for transfer. The copy sheet carrying the transferred toner image is led to a fuser to fix toner to the sheet, and thus, a copy is obtained.
A density of an image formed on a sheet varies depending upon a degree of the exposure of the photoconductor drum, a degree of the electrostatic charge on the photoconductor drum, a developing bias, etc. Utilizing such a phenomenon, the density of the image formed on the sheet can be adjusted. In an arrangement for adjusting the developing bias to regulate the density, for example, the developing bias is varied in accordance with a density set by an operator or a density of an original automatically sensed.
In order to use several separate copying machines to obtain uniform images corresponding to a density which is set by an operator, for example, it is necessary adjust parameters in connection with a photoconductor drum in advance for each copying machine, where "parameters" are image forming conditions such as a degree of exposure of the photoconductor drum, a degree of electrostatic charge, a developing bias, etc. Thus, at the last stage of producing copying machines or upon a maintenance check of the copying machines, various parameters related to each photoconductor drum must be adjusted to make a uniform density for a copy image formed by each copying machine.
In typical prior art technology for adjustments in the above-mentioned situation, a photosensor is placed relative to the photoconductor drum so as to sense a density of a toner image formed on a surface of the photoconductor drum. When each parameter in connection with the photoconductor drum is to be adjusted, a reference original which contains an image of a reference density is copied on trial. An engineer conducting adjustments decides if the density of the toner image sensed by the photosensor conforms to the reference density. Judging from a result, the engineer regulates each parameter in connection with the photoconductive drum.
Even if a copy image of a desirable density is once obtained based upon a reference original of a certain density, a copy image of an adequate density is not always obtained based upon a reference original of another density. Then, reference originals at several density levels from a fog density corresponding to a density of the background of an image (i.e., white) to a solid density corresponding to a solid image (i.e., black) are copied on trial, and parameters in connection with the photoconductor drum are adjusted so that an image at any density level from the fog density to the solid density can be reproduced well. After such adjustments, an image of any density in an original can be copied into a well-reproduced image.
For the photosensor for detecting a density of a toner image, a reflection type photosensor is generally applied. The reflection type photosensor includes a light emitting element and a light receiving element put in position opposed to the photoconductor drum. The light emitting element emits a fixed quantity of light and the light receiving element receives reflected light from the photoconductor drum to sense the quantity of the reflected light. Output from the light receiving element corresponds to a density of a toner image on a surface of the photoconductor drum.
FIG. 13 is a diagram illustrating an output characteristic of the above-mentioned reflection type photosensor. In FIG. 13, there are shown outputs from the reflection type photosensor when the reference originals at various density levels are copied respectively under fixed conditions of a degree of exposure of the photoconductor drum, a degree of electrostatic charge, the developing bias, etc. For a reference original, a Munsell original is used. A density value rising as the density increases is applied to a Munsell original. For example, density value "0" corresponds to a white original while density value "10" corresponds to a solid black original.
As can be seen in FIG. 13, the outputs from the reflection type photosensor vary in a relatively linear manner in a region of gray or medium densities. Hence, the reflection type photosensor can sense differences in density; that is, it can be determined well if there is any difference between the density of actual toner images and the reference density based upon the outputs from the reflection type photosensor.
However, regarding the fog density and the solid density, it is likely to misjudge an adequate density of the toner image. The fog density corresponds to the density value "0" while the solid density corresponds to the density value "10". In such extreme density regions, the outputs from the reflection type photosensor are saturated, as shown by alphanumeric symbols A1 and A2 in FIG. 13. Regarding the fog density and the solid density, there is less variation in the outputs from the reflection type photosensor. Hence, it cannot be determined with good accuracy if there is any difference between the density of the toner image and the reference density. This may cause maladjustment of the density.
To overcome an above-mentioned disadvantage, it may be proposed that a reflection type photosensor having an output characteristic linear in any density region; however, it is difficult to realize such an output characteristic because of features of the sensor.
Then, there may be an improvement that a variable resistance is connected to the light emitting element and the light receiving element constituting the reflection type photosensor to alter the output characteristic of the reflection type photosensor; that is, an operative point and a sensitivity of the sensor are altered by varying a resistance value of the variable resistance.
An output characteristic of the reflection type photosensor after adjustment of the variable resistance is shown in FIG. 14. The fog density can be detected well under the output characteristic expressed by a curve L1 while the solid density can be detected well under the output characteristic expressed by a curve L2.
In such an arrangement, however, the reflecting type photosensor is connected with the variable resistance particularly provided for detection of the fog density and the solid density, so that there arises the disadvantage that the arrangement is complicated and therefore is costly in manufacturing.